In recent years, the vehicle is used under environments of a high temperature and a low pressure, and a fuel high in vapor pressure such as an alcohol mixed fuel is used for the fuel, potentially resulting in a state where vapor is likely to be generated in the fuel to be supplied from the fuel tank into the internal combustion engine. In this case, the vapor corresponds to air bubbles.
Patent Literature 1 discloses a technique in which when vapor is generated in the fuel flowing in a fuel pipe that connects the fuel pump and the internal combustion engine, a target fuel pressure of the fuel discharged by the fuel pump is set to be higher to prevent vapor lock from being generated in the fuel pipe.
Incidentally, in recent years, in order to reduce a power consumption of the vehicle, a variable fuel pressure system that pumps a fuel corresponding to a fuel pressure and a flow rate required by the internal combustion engine into the internal combustion engine from the fuel pump may be employed as a control system of a fuel supply system. A stable discharge at a low flow rate is required for the fuel pump used for the system.
However, when vapor is generated in the fuel in a pump chamber that boosts a fuel pressure in the fuel pump when the fuel pump discharges the fuel at the low flow rate, it is difficult to discharge the vapor from the pump chamber together with the fuel.
In this case, even when the target fuel pressure of the fuel discharged by the fuel pump is set to be higher through the technique disclosed in Patent Literature 1, it is difficult to discharge the vapor from the pump chamber by the rotation of an impeller driven according to the higher target fuel pressure under control.
If a large amount of vapor is stored in the pump chamber of the fuel pump, there is a risk that the fuel pump is vapor-locked, and the fuel is not discharged.